A method for protecting an electronic component or an electronic circuit from overvoltage or static electricity is known, which an overvoltage protector is connected to a ground and one of an input terminal and an output terminal of the electronic component or apparatus to be protected. Such overvoltage protector functions as an insulator under a normal operation condition. Upon having an overvoltage applied, the protector has impedance greatly decreasing or discharges electricity inside the protector to become electrically conductive.
An example of the overvoltage protector includes an overvoltage protection material between a pair of electrodes is known. The overvoltage protection material functions as an insulator under a normal operation condition, but passes an overvoltage once the overvoltage is applied to it.
As one the overvoltage protection material, material containing metallic particles with a passive state layer formed thereon and resin mixed with the particles is known. (cf. Patent Literature 1)
As another overvoltage protection material, material containing one of a conductor and a semiconductor having a high aspect ratio dispersed into an aggregate in nanoscale and another one of conductive or semi-conductive material added thereto is known. (cf. Patent Literature 2)
As still another overvoltage protection material, material containing inorganic conductive material having 1 nm to 200 nm particle diameter dispersed discontinuously into insulating material is known. (cf. Patent Literature 3)
In recent years, an overvoltage protector having a high reliability is increasingly demanded. Specifically, a highly reliable overvoltage protector operating correctly upon receiving a high voltage or a repetitive overvoltage is demanded. This is because a required reliability level for electronic devices or electrical apparatus installing an overvoltage protector became high in every service place, application and use environment. This is also based on an expectation to enhance a safety factor or to secure a wide safety margin in the electrical apparatus by requesting the overvoltage protector to have a much higher reliability than was ordinarily requested. A severe reliability test is conducted in which a much higher voltage is applied repeatedly or a short pulse high voltage is successively applied.
Applying repetitive high voltage short pulses is a much rigorous test than applying an ordinary static electricity. But, such test is sometimes conducted to secure a higher safety factor of the electrical apparatus against overvoltage. When conducting such tests, however, metal particles in the overvoltage protection part formed between the pair of discharging electrodes of the overvoltage protector occasionally melts, causing a short circuit problem.